1. Field of the Invention
The present invention generally relates to a switching circuit for and a method of electronically selecting between a pair of audio input signals in an audio system and, more particularly, to a noise-free switching circuit operative for reducing audible noise during switching in accordance with the method of the present invention.
2. Description of the Prior Art
It is known in the art of electronic switching circuits to use switching transistors to select one of a pair of audio input signals in an audio system. Conventionally, the control voltage to be applied to the input of each switching transistor in a step voltage. However, the rise and fall time characteristics of such transistors are typically not ideal and, therefore, such transistors having non-linear operating characteristics are not necessarily turned on and off simultaneously with the abrupt changes in voltage level with the step voltage.
Thus, the input step voltage will generate a square wave-like output voltage having overshoot and undershoot, variations which are primary noise-contributing factors in an acoustic system. As is well known, a step or pulse-like waveform of short duration contains frequency components over the complete frequency spectrum, and the frequency components in the audible frequency range generally have a higher amplitude than the frequency components above the audible range. Thus, the overshoot and undershoot voltage variations produce noise which is particularly objectionable and offensive to the ear when generated in an acoustic apparatus.
It is also known in the prior art switching circuits to employ a pair of audio signal-bearing wires to conduct a respective audio signal from a respective input terminal to an output terminal. To effect switching, a manually-operated mechanical switch is electrically connected to both signal-bearing wires. However, separate expensive shielded wiring is required to span the distance between the signal-bearing wires at the interior of an acoustic apparatus and the mechanical switch mounted on the front face of the acoustic apparatus. Moreover, switching performed directly in the signals paths produce very objectionable clicking and hissing noises.